The detection of concentrations of various gaseous species in ambient air and in other environments with varying temperature and/or pressure conditions has become ever more important. Potential applications include industrial process streams such as dryers, process columns, or combustors, as well as monitoring of toxic species in ambient environments. Diode laser systems have been used to measure these various gaseous species. In these systems a laser beam is directed through a sample region containing a gas or gases, each of which absorb laser energy at particular wavelengths. The type of gas or gases in the region can be determined by noting the wavelength(s) of the energy absorbed or more typically by supplying the region with a known wavelength which corresponds to the absorption wavelength of a particular gaseous species of interest. The concentration of the gas can be computed from the pressure, temperature, and the amount of absorption, which may be accomplished by comparing the amount of incident radiation to the amount of transmitted radiation: the greater the absorption, the greater the species concentration.
Maintaining a lock on and continuously outputting a laser wavelength equal to the absorption wavelength of a particular species being monitored is crucial to obtaining accurate concentration measurements. However, this is difficult due to laser wavelength drifts away from coincidence with the species absorption wavelength. To ensure accurate real-time concentration monitoring under changing conditions a wavelength-locked laser gaseous species monitor system, as disclosed in U.S. Pat. No. 5,026,991 (the '991 patent), was developed. That patent is incorporated herein by reference in its entirety. The system disclosed in that patent utilizes a reference cell containing a known concentration of the gaseous species to be monitored in a feedback loop so that the laser wavelength (corresponding to the species absorption wavelength) can be continuously monitored and adjusted over time in order to lock it to the absorption line wavelength of the species being monitored.
Certain species, however, cannot be contained in a reference cell because such species react with or corrode the reference cell. For example, HF will corrode the optics of the cell. Also, unstable or transient species, such as OH and CH, are not viable under ambient reference cell conditions. These species exist only under certain pressure and temperature conditions. Finally, internally excited states of stable species may have very low concentrations at ambient reference cell conditions. Nevertheless, it is still desirable to measure the concentrations of these types of species accurately.
One method that does not require a reference cell but is capable of measuring the concentration(s) of species not capable of being contained in a reference cell is Fourier Transform Infrared (FTIR) spectroscopy . However, this method is not suitable for making measurements in environments where species concentration are varying rapidly over time. Moreover, systems for performing FTIR spectroscopy are large, expensive and slow.
Another method not requiring the containment of a sample in a reference cell of the gaseous species to be measured is laser diode spectroscopy. With this method the laser is scanned over the entire spectrum and is not locked to a single absorption line. Because of this the system is slow, requiring scanning of the entire spectrum even though only a single wavelength may be of interest. Moreover, this method is not very sensitive and it makes monitoring concentration variations very difficult as the species is not being continuously monitored.
Spectroscopy with a broad band radiation source does enable monitoring of species without a reference cell. However, since the band pass is not narrow it is susceptible to interfering species, and it is less sensitive.